In recent years a need arose for coating compositions that function to replace chromates in metal treatment. This is due to the detrimental health and environmental impact that has been determined to be associated with chromium compounds in particular. There is also a need for an alternative replacement coating that is formed from an aqueous solution. This eliminates the disposal and emission considerations involved in producing zirconates and other metal oxide-containing coatings from sol-gel applications, while providing a broad-spectrum replacement for undesirable metal treatments such as chromates and molybdates.
There are believed to be several mechanisms by which chromates provide protection to an underlying substrate. While the complete source of the protection has not been fully elucidated, there has been considerable research to identify each aspect of the chromate mechanistic model. In Corrosion Science, 34 (1), 41 (1993), Kendig, Davenport and Isaacs used XANES to demonstrate variable valence states of chromium in chromate coatings. This revealed both the +3 and +6 oxidation states. The chromium in both states is present as oxides. The +3 state forms a stable "long-range" oxy-polymer and the chromium remaining in the +6 state, which is trapped in the film, has limited long-range structure.
The protection would then come from at least two mechanistic aspects. One is the physical aspect of protection provided by the stable +3 oxide matrix. A secondary protective source is the +6 chromate in the film. The trapped reservoir of +6 chromate is in some way available to heal the film in some fashion once corrosive attack begins.
Many chromate-free chemical conversion coatings for metal surfaces are known to the art. These are designed to render a metal surface "passive" (or less "reactive" in a corrosive environment), leaving the underlying metal protected from the environment. Coatings of this type that produce a corrosion resistant outer layer on the base metal or its oxide often simultaneously produce a surface with improved paint adhesion. Conversion coatings may be applied by a no-rinse process, in which the substrate surface is treated by dipping, spraying, or roll coating. The coatings may also be applied in one or more stages that are subsequently rinsed with water to remove undesirable contaminants.
Several metal and metaloid elements will form a continuous three-dimensional polymeric metal- or metaloid-oxide matrix from aqueous solutions. Chromium shares this characteristic along with silicon and other elements. The Group IV-A elements continue to be attractive candidates for chromate replacement technologies as they share the virtue of being relatively innocuous environmentally and have common valences of +4, facilitating the formation of three dimensional amorphous coatings.
Non-chrome conversion coatings are generally based on chemical mixtures that in some fashion will react with the substrate surface and bind to it to form protective layers. The layer or layers may yield protection through galvanic effects or through simply providing a physical barrier to the surrounding environment.
Many of these conversion coatings have been based on Group IV-A metals such as titanium, zirconium and hafnium, a source of fluoride and a mineral acid for pH adjustment. Fluoride has typically been considered to be necessary to maintain the Group IV-A and other metals in solution as a complex fluorides. The fluoride may also serve to keep dissolved substrate metal ions (such as aluminum) in solution.
For example, U.S. Pat. No. 4,338,140 to Reghi discloses a coating for improved corrosion resistance with solutions containing zirconium, fluoride and tannin compounds at pH values from 1.5 to 3.5. Optionally, the coating may contain phosphate ions.
U.S. Pat. No. 4,470,853 to Das is related to a coating composition comprised of zirconium, fluoride, tannin, phosphate, and zinc in the pH range of 2.3 to 2.95. According to Das, it is important that approximately 10 atomic percent of zirconium-zirconium oxide be present in the coating to obtain "TR-4" corrosion resistance. It was shown that coatings of higher zirconium oxide content produced excellent corrosion resistance. Compositions which gave higher zirconium oxide on the surface were preferred in the disclosures.
U.S. Pat. No. 4,462,842 to Uchiyama and U.S. Pat. No. 5,380,374 to Tomlinson disclose zirconium treatments in solutions containing fluorides which are followed by treatment with silicate solutions. This combination is suggested to form zirconate and syloxyl linkages (--O--Zr--O--Si--O--Si-- . . . ), yielding a coating with improved corrosion resistance over the zirconium treatment alone. Coatings of this type give excellent corrosion protection but very poor paint adhesion.
The compositions and processes of Uchiyama are useful in producing hydrophilic surfaces. The compositions of Tomlinson purportedly do the same when subsequently treated per Uchiyama. The compositions of Tomlinson are high in Group II-A metals, which somewhat improve the latent corrosion protection of the fluoro-Group IV-A coating formed. The drawback is that the solubility of Group II-A components is limited, therefore the opportunity to formulate stable concentrates may not be possible.
Additionally, coating compositions high in the Group II-A elements tend to generate considerable scaling as described by Reghi in U.S. Pat. No. 4,338,140. While an incremental improvement in paint adhesion may be afforded by Group II-A metal inclusion in some aspect of the present invention, they may actually inhibit formation of the continuous amorphous metal oxide matrices in some cases.
In Reghi and in U.S. Pat. Nos. 5,380,374 and 5,441,580 to Tomlinson, Group I-A and Group II-A elements probably incorporate as "discrete," non-bonded cations, perhaps providing some space-charge stabilization to balance discrete anions in the coatings. But these compositions likely provide little if any long-range structure.
U.S. Pat. No. 4,863,706 to Wada discloses a process for producing sols and gels of zirconium and a process for producing zirconia. The processes described include reactions to produce basic boratozirconium and basic boratozirconium chloride sols. These were purportedly used in producing boratozirconium and boratozirconium chloride gels. Further described is a method for producing zirconia from the gels at relatively low temperature. The essential components include a boron compound along with a polyvalent metal, zirconium and chloride.
U.S. Pat. No. 5,397,390 to Gorecki discloses an adhesion promoting rinse containing zirconium in combination with one or more organosilanes and fluoride. The compositions are used to rinse surfaces after they have been treated in a phosphating bath. The zirconium ion concentration is selected to maintain pH in a broad range as the silanes deposit on the substrate to promote paint adhesion and improve corrosion resistance. Organosilanes are necessary components of the disclosed compositions. Additionally, in preparing the compositions, Gorecki indicates that whenever zirconium-containing salts such as zirconium basic carbonate, zirconium hydroxychloride and zirconium oxychloride are used as a source (of zirconium) the salts must be dissolved in 50% hydrofluoric acid in order to effect dissolution. Gorecki does not indicate a necessity to dissolve the fluorozirconate salts mentioned in his disclosure. This demonstrates that fluoride is a necessary component of the disclosed compositions as it is included as part of the fluorozirconate salts or from hydrofluoric acid.
Brit. Pat. 1,504,494 to Matsushima describes a process for treating metal surfaces using zirconium at a pH above 10.0. A zirconate coating is formed but the pH of the solution is maintained above the present invention.
U.S. Pat. No. 5,603,754 to Aoki describes the use of zirconium and titanium ions in the presence of fluorides, oxidizing agents and aluminum and other components. The coatings appear to be mixed fluoro-forms of tin, aluminum, zirconium or titanium phosphates. The coatings appear to provide an excellent surface for painting or printing. Fluorozirconates and fluorotitanates are used, indicating a high fluoride to Group IV-A metal ratio.
U.S. Pat. No. 5,759,244 to Tomlinson discloses compositions with fluoride to Group IV-A metal at a molar ratio in the range of less than or equal to two to one and zero to one. The compositions are effective in providing corrosion resistance to many alloys.
U.S. Pat. No. 5,760,112 to Hirota describes an organic coating with carbon black as a pigment, oxidizing ions and, optionally, metal ions. The organic polymer formed from the dispersion upon curing is fundamentally different from the films provided in the present disclosure. But the present invention would provide an inorganic alternative to such compositions in the same pH range using the same application techniques.
One avenue of research into protecting the copper bearing aluminum alloys has been to provide compositions that contain azole derivatives to complex any copper that dissolves during corrosive attack. This can happen through various cells that can be established at copper inclusions at the surface of these alloys. U.S. Pat. No. 5,128,065 to Hollander discloses this type of chemistry. The azoles of this type and some of those disclosed by Cha in U.S. Pat. No. 5,156,769 show some promise.
In addition, many health and environmental benefits of eliminating or reducing fluoride have been addressed in systems based on chemistries other than those of the Group IV-A metals. Examples are described in UK Pat. Application 2,084,614 by Higgins.
In view of the foregoing, it can be seen that there exists a need for an improved "broad-spectrum" coating which can be used in a number of applications, and which is also environmentally sound and has a low impact in the workplace. It will be appreciated that there exists a need for broad-spectrum coating systems which are aqueous, promote paint adhesion and provide environmental resistance simultaneously.
It is an object of the present invention to provide such compositions, as well as certain processes for coating substrates that incorporate said compositions. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.